Fuel cell assemblies employing a plurality of individual fuel cell modules are well known. Each module has an anode and a cathode. In a proton-exchange fuel cell, the anode and cathode are separated by a catalytic proton exchange membrane (PEM) in which the modules in the stack typically are connected in series electrically through bipolar plates to provide a desired total output voltage. Fuel in the form of hydrogen and water vapor, or hydrogen-containing mixtures such as “reformed” hydrocarbons, is flowed through a first set of reaction channels formed in a first surface of the bipolar plate adjacent the anode. Oxygen, typically in the form of air, is flowed through a second set of reaction channels formed in a second surface of the bipolar plate adjacent the cathode.
In a PEM fuel cell, hydrogen is catalytically oxidized at the anode-membrane interface. The resulting proton, H+, migrates through the membrane to the cathode-membrane interface where it combines with ionic oxygen to form water. Electrons flow from the anode through a load to the cathode, doing electrical work in the load.
In fuel cells, a long-term electrical continuity problem is well known in the art. Metals typically used to form bipolar plates, for example, aluminum or stainless steel, can corrode and/or form high-resistance oxide passivation layers on the surface of the bipolar plates because of electrochemical activity at these surfaces. These high resistance oxide layers can inhibit corrosion, but they may also limit the current-collecting ability of the bipolar plates, thereby significantly lowering the efficiency and output of a fuel cell. In the prior art, bipolar plates are known to be coated with noble metals such as gold and platinum to prevent corrosion and the formation of high resistant passivation layers on the electrical contact surfaces, but such coatings are so expensive as to impact the widespread use of cost-effective fuel cells. Other approaches to improving the electrical interconnects of fuel cells are described in the following patents, the disclosures of which are all incorporated herein by reference:
U.S. Pat. No. 6,805,989 discloses a separator for a solid polymer electrolyte fuel cell that comprises a cladding material that covers a highly conductive metal with highly corrosion-resistant titanium or titanium alloy, at least a portion of which is covered by a carbon material.
U.S. Pat. No. 6,843,960 discloses a method for making metal plates for planar solid oxide fuel cells from powders of predominantly iron alloys that also include small amounts of chromium, lanthanum, yttrium, and strontium.
U.S. Pat. No. 6,280,868 discloses an electrical interconnect device for a planar fuel cell that comprises a chromium-containing substrate having on the anode-contacting side an oxidation-resistant coating that comprises an outer oxygen barrier layer comprising nickel, a noble metal other than silver, or an alloy of these metals and an electrically conducting metal barrier layer comprising niobium, tantalum, silver, or an alloy of these metals between the substrate and the upper layer.
U.S. Pat. No. 5,942,349 discloses an electrical interconnect device for a planar fuel cell that comprises a chromium-containing substrate having on the cathode-contacting side a coating comprising an oxide surface layer comprising at least one metal M selected from Mn, Fe, Co, and Ni, and an M, Cr spinel layer between the substrate and the oxide surface layer.
U.S. Pat. No. 6,620,541 discloses a high temperature fuel cell comprising an electrolyte/electrode unit having an anode, an interconnector having a fuel gas side, and first and second metallic functional layers applied one above the other on the fuel gas side of the interconnector, the first functional layer containing nickel and the underlying second functional layer containing copper, the first functional layer being connected to the anode by an electrical conductor.